Field of the Invention
The invention concerns a method for the correction of image data dynamically acquired with a magnetic resonance imaging method. The invention also relates to an image correcting device for a magnetic resonance scan. The invention also relates to a magnetic resonance system.
The invention makes reference to co-pending U.S. application Ser. No. 14/932,235, filed Nov. 4, 2014, corresponding to German patent application no. 102014222495.0. The content of the co-pending United States application is incorporated herein by reference.
Description of the Prior Art and Related Subject Matter
In what is known as Echo Planar Imaging (EPI) a number of phase-encoded echoes is used to fill a slice in k-space, also called a raw data matrix. The basic idea of this technique is to generate a series of echoes in readout gradients, after an individual (selective) RF excitation, which is allocated by a suitable modulation of the phase-encoding gradient to different lines in k-space plane. In this way, for example, a whole slice can be detected with a single RF excitation.
Due to the long echo train, particularly in the case of echo planar imaging additional geometric distortions, such as elongations or compressions, occur in the image. These will be called distortions below.
As described in detail in the aforementioned co-pending application Ser. No. 14/932,235, these artifacts can be eliminated inter alia by retrospective methods. These intervene in the reconstruction process only after recording of the raw data. With this type of post-processing method a separate field map, also called a B0 field map, is conventionally recorded which may be present either in undistorted coordinates or in distorted coordinates.
B0 field maps indicate the spatial distribution of what are known as off-resonance frequencies. Off-resonance frequencies should be taken to mean frequency shifts of the scan signals which occur due to the susceptibility jumps and external magnetic field inhomogeneities.
As described in detail in co-pending application Ser. No. 14/932,235, there exist both various types of B0 field maps (distorted and undistorted) and a wide variety of correction algorithms. These algorithms are each fixed to a one type of B0 field map. Since both field map types and the correction algorithms have different advantages and disadvantages, a flexible combination of any field maps with any algorithms is desirable. One possibility of converting field maps into each other is described in detail in co-pending application Ser. No. 14/932,235.
Due to external influences, heating of the system or patient movement the actual field distribution with respect to the original distribution detected in a field map can change with time. It is precisely with relatively long recording periods that dynamic updating of the field maps is therefore desirable, so that the geometric distortion correction does not lead to incorrect results.
A dynamic correction with the use of distorted field maps can be implemented in that one up-to-date field map respectively is created during the course of a continuous EPI recording from two repeated slice recordings respectively with the aid of the PLACE method. Since the image phase in the two slice recordings is affected differently by said dynamic influences, for example breathing, and the PLACE method is based on the phase difference between the two recordings, inaccuracies can occur during recording of the updated B0 field maps.
If undistorted B0 field maps are acquired on the basis of gradient echo recordings, admittedly these inaccuracies do not occur, but gradient echo recordings are not suitable for integration in a continuous image recording since they require a great deal of time. For this reason undistorted field maps are only carried out as a pre-scan, but are not suitable for carrying out an update during the image recording.